Today a number of different technologies for cellular telecommunication exist. One such existing cellular telecommunication technology is Wideband Code Division Multiple Access (WCDMA).
In a WCDMA system, which is a powerful standard able to transmit data over a radio network at very high speed, a User Equipment (UE) such as a mobile telephone communicates over radio channels with base stations typically denoted Node B. The omni-area around a base station can be allocated into several sectors known as cells. The base station transmits and receives signals over selected radio channels in each cell. Typically, a base station is connected to one or more radio network controller nodes (RNC). One or more RNCs are, in turn, connected to the core network (CN). The CN is usually connected. e.g., via a gateway to other telecommunication networks, such as the public switched telephone network or to a packet-data network such as the internet.
In a wideband code division multiple access (WCDMA) mobile telecommunications system, the information transmitted between a base station and a particular UE is modulated by mathematical codes (such as spreading codes) to differentiate the information for different services of this UE and the information for different UEs which are utilizing the same radio frequency. Thus, in WCDMA the data signal over each mobile radio employs its own unique code sequence to encode its signal. The receiver, knowing the code sequences of the mobile radio it services, decodes the received signal to recover data from each radio.
In the standard 3GPP Release. 5, High-Speed Downlink Packet Access (IISDPA) is introduced for WCDMA. MSDPA achieves the increase in the data transfer speeds by defining a new WCDMA channel: a high-speed downlink shared channel (HS-DSCl-1) that operates in a different way from existing DPCH channels and is used for downlink communications to the mobile.
Along with the 1-1s-DSCI 1 channel, three new physical channels are also introduced. One is the High Speed-Shared Control CHannel (HS-SCCH) which informs the user that data will be sent on the HS-DSCH 2 slots later. The second one is the uplink High Speed-Dedicated Physical Control CHannel (HS-DPCCH), which carries acknowledgement information and current channel quality indicator (CQI) of the user. This value is then used by the Node-B in scheduling, including scheduling user and calculating how much data to send to the scheduled UEs. The third downlink physical channel is the High Speed-Physical Dedicated Shared CHannel (HS-PDSCH), which carries the information transferred by HS-DSCH.
Furthermore, TTI, Transmission Time Interval, is a parameter in WCDMA and other digital telecommunication networks related to encapsulation of data from higher layers into frames for transmission on the radio link layer. TTI refers to the length of an independently decodable transmission on the radio link. The TTI is related to the size of the data blocks passed from the higher network layers to the radio link layer.
To combat errors due to fading and interference on the radio link the data in the transmitter buffer is divided into blocks and then the bits within a block are encoded and interleaved. The length of time required to transmit one such block determines the TTI. At the receiver side all bits of a given block must be received before they can be deinterleaved and decoded.
In order to be able to adapt quickly to the changing conditions in the radio link a communications system must have shorter TTIs. In order to benefit more from the effect of interleaving and to increase the efficiency of error-correction and compression techniques a system must, in general, have longer TTIs. These two contradicting requirements determine the choice of the TTI.
In 3GPP Release '99 the shortest TTI is 10 ms and can be 20 ms, 40 ms, or 80 ms. In 3GPP Release-5 the TTI for HSDPA is reduced to 2 ms. This provides the advantage of faster response to link conditions and allows the system to quickly schedule transmissions to mobiles which temporarily enjoy better than usual link conditions.
In FIG. 1, the timing of HSDPA transmission in the air interface is depicted. The control information for a UE is sent over the HS-SCCH 2 slots prior to the corresponding data transmission over the HS-DSCH in order to ensure that the UE has enough time to receive and decode the necessary information. Based on this information the UE determines if and how to receive the subsequent HS-DSCH data. If the UE determines that there is an HS-DSCH carrying data for that particular UE, the CE receives the HS-DSCH data and starts to process the received data as soon as the HS-DSCH receiving ends.
The UE takes about 7.5 slots to process the received HS-DSCH. Then the acknowledgement information for the HS-DSCH is sent over the first slot of HS-DPCCH. The duration from the HS-SCCH transmission start to the HS-DPCCH transmission end is about 15.5 slots and can not be interrupted, otherwise the data could be regarded as being lost.
In order to perform Inter Frequency Handover (IFHO) and Inter Radio Access Technology Handover (IRAT HO), ComPressed Mode (CPM) for HSDPA is to be implemented. FIG. 2 shows the timing of the CPM timing in the air interface. The CPM is defined in “3GPP TS 25.215 V6.4.0, Technical Specification Group Radio Access Network; Physical layer—Measurements (FDD)”. The following abbreviations will be used: TG: Transmission Gap. TGPL: Transmission Gap Pattern Length in number of frames. TGCFN: Transmission Gap Connection Frame Number, which is the Connection Frame Number (CFN) of the first radio frame of the first pattern within the TG-Pattern sequence. TGSN: Transmission Gap Starting Slot Number, which indicates the time offset from the TGPL start to the transmission gap start. TGL: Transmission Gap Length in number of slots. TGD: Transmission Gap start Distance in number slots, which is the duration between the starting slots of two consecutive transmission gaps within one transmission gap pattern.
Furthermore, there are different TG-Patterns for different measurement targets. Here are some examples:
The current default TG-Pattern for IFHO:
TGPL=4, TGL=7, TGSN=4, TGD=0, TGCFN=0
The current default TG-Patterns for IRAT HO:
TGPL=8, TGL=7, TGSN=4, TGD=0, TGCFN=0
TGPL=8, TGL=7, TGSN=4, TGD=0, TGCFN=2
TGPL=8, TGL=7, TGSN=4. TGD=0, TGCFN=6
There are also some proposed TG-Patterns namely:
TGPL=4, TGL=14, TGSN=8, TGD=0, TGCFN=0
TGPL=8, TGL=14, TGSN=8, TGD=0, TGCFN=0
TGPL=24, TGL=14, TGSN=8, TGD=0, TGCFN=4
TGPL=24, TGL=14, TGSN=8, TGD=0, TGCFN=13
TGPL=24, TGL=14, TGSN=8, TGD=0, TGCFN=20
During the TG, the UE is performing measurements in another frequency in the original network or another network with different radio access technology (RAT). Hence, the UE can not transmit signal to or receive the signal from the original serving cell during this time.
The impact of the CPM on HSDPA users will now be described. As the process from the HS-SCCM transmission start to the corresponding HS-DPCCH transmission end can not be interrupted, scheduling a user in CPM should be avoided during a gap described above and the 15.5 slots before the gap. For a 7-slot or 14-slot gap, the number of the usable slots is then about 22.5 or 29.5 slots. The UE may work in multiple TG-Patterns. Also, depending on the TGD setting, there are possibly several gaps within one pattern.
Furthermore, the UE can be in CPM for several seconds or even longer, during which the UE suffers a longer packet delay than before the UE entered CPM.
Another factor that has to be considered is that the UE in CPM usually suffers very bad channel quality. For some schedulers such as proportional fair or maximum channel quality indicator (CQI), which considers the channel quality, the scheduling delay for the UEs in CPM is very large even if the impact of the gap is not taken into account.
Hence, the UEs in CPM experience a very large packet delay compared to those UEs not in the CPM due to both the bad channel quality and the transmission gap. The UEs in CPM also have a high risk of experiencing a high packet loss because of the retransmission failure resulted from T1 timer expires and the maximum scheduling delay reached if there is a maximum delay threshold setting in the scheduler such as the High Speed Medium Access protocol (MAC-hs) delay scheduler.